Constant Speed Tach Time

R

rajsingh

New Member
I had a question regarding tach time on aircraft using a constant speed propellar. I know many clubs that charge tach time which is great. But what about the aircraft that have a constant speed propeller. Can't you theoretically get the same power output on a lower rpm. What if you lower the rpm and increase manifold or does that over stress the engine and you shouldn't do it. Do you have to increase the rpm when you increase the power, and lower rpm when you lower the power. Because after understanding little bit about how the constand speed prop works, I wondered that wouldn't somebody just lower the rpm to save on tach time. But I have heard at the same time that when you increase power, you increase the rpm as well, and when you lower the power you lower the rpm.

Any thoughts!
 
rajsingh said:
Any thoughts!
Click to expand...

You're correct that for a given percent power, there are many different settings that can achieve it, at least for percents less than 100. That's due to the fact that power = torque * rpm; a large torque can compensate for a low rpm. Torque will be high with a course prop setting and high throttle position. Your POH should show you several settings to accomplish the same power.

The engine manufacturers place limits on how high you can take manifold pressure when you have low rpm settings; unless you have the engine documentation, you don't know where those limits are, so be careful to stick to the combinations listed in the POH.

 
The Tach also stops spinning faster, similar to the vacuum pump, around 1500 RPM. Above that it will display the proper time while below is goes at a much slower rate.
 
I didn't know that a tach turned into a clock after a certain rpm. I actually thought that it would run faster during faster and be normal around cruise speed.
 
The tach will continue to spin faster and faster with the engine, right up until the engine comes apart ;)

What you're thinking of is that each tach hourmeter comes calibrated to turn 1:1 at a certain RPM. Most of the ones I've seen are around 2500 RPM. Below that it'll tick over slower, above that it'll tick over faster. So yes, using low RPM will make the tach turn slower, most places, though, charge 1.2 x tach to make up for that.

The vacuum pump is the same also, I assume what you mean when you say it "stops spinning faster" is that the vacuum needle comes up into the green and stays there. It's because the vacuum system has a pressure regulator in it to keep it there, not because the pump stays at one speed.
 
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